tert-Butanol (TBA) is an important product produced on a large industrial scale and is used as solvent and as intermediate for the preparation of methyl methacrylate. It is a precursor for the preparation of peroxides such as peroxy ketals, peresters or dialkyl peroxides having at least one tertiary butyl group. These compounds are used as oxidants and as initiators for free-radical reactions, for example olefin polymerization or crosslinking of plastics. tert-Butanol serves as intermediate in the isolation of pure isobutene from isobutene mixtures. Furthermore, it is a reagent for the introduction of tertiary butyl groups. Its alkali metal salts are strong bases which are used in many syntheses.
Tertiary butanol can be obtained by acid-catalyzed addition of water onto isobutene. Industrial isobutene mixtures frequently further comprise other olefins such as 2-butenes. If these starting materials are used, industrial processes employ conditions in which virtually exclusively the isobutene but not the other olefins are hydrated and secondary reactions such as homooligomerization or heterooligomerization of the olefins are virtually completely suppressed. Such processes are usually carried out in the liquid phase and can be divided into two groups: a) processes in which the reaction is carried out in an aqueous catalyst solution and b) heterogeneously catalyzed processes in which solid catalysts which are insoluble in the reaction phase are used.
Homogeneously catalyzed processes employ sulfuric acid, heteropolyacids, p-toluenesulfonic acid or other strong acids as catalyst. These highly active homogeneous catalysts usually form a homogeneous phase with the reaction product, so that the catalyst cannot be separated off mechanically. In some processes, a solvent is additionally used. If the tertiary butanol is isolated from the reaction solution by distillation, the yield is reduced by the backreaction and the formation of by-products.
Heterogeneously catalyzed processes are frequently carried out using acidic ion exchangers as catalysts.
EP 0 579 153 describes a process in which an aqueous solution of a sulfonated polymer (polystyrenesulfonic acids or polyvinylsulfonic acids having molar masses of from 1 000 to 100 000 g/mol) is used as catalyst. Isobutene or the isobutene-containing starting material and the catalyst phase flow concurrently through the reactor. The output from the reactor consists of two phases and separates into a tert-butanol-containing organic upper phase and the catalyst phase. The organic phase is worked up by distillation to isolate tert-butanol. Water is added to the catalyst phase to replace the water which has been consumed and the catalyst phase is then recirculated to the reactor.
The hydration of isobutene to form tert-butanol with the aid of solid acidic catalysts which are soluble neither in the starting materials nor the products has the advantage that the reaction mixture is free of acid and can be worked up to isolate tert-butanol without losses caused by redissociation or other secondary reactions. The reaction proceeds at the surface of the catalyst. For a reaction to occur, both reactants have to be present simultaneously at the active site of the catalyst. This is made relatively difficult by water and isobutene or an isobutene-containing hydrocarbon mixture not being miscible with one another. To obtain acceptable conversions, use is made of solvents which make it possible to obtain a homogeneous mixture of water and isobutene feed mixture.
DE 30 31 702 describes the use of methanol as solvent for this purpose both for water and for isobutene or an isobutene-containing hydrocarbon mixture. tert-Butanol and methyl tert-butyl ether are obtained side by side as products.
In EP 0 010 993, aliphatic carboxylic acids having from 1 to 6 carbon atoms are used as solvents for both starting materials. The tertiary butyl esters of these acids are formed as by-products and have to be hydrolyzed to tert-butanol and carboxylic acids.
DE 030 31 702 uses sulfolanes and U.S. Pat. No. 4,327,231 uses polyhydric alcohols of the neo type, for example neopentyl glycol. These solvents have to be separated off from the tert-butanol. In addition, there is a risk of the solvent used being decomposed in long-term operation of such a plant.
WO 99/33775 describes a process for preparing tert-butanol by reaction of a mixture comprising water, tert-butanol and isobutene or an isobutene-containing hydrocarbon mixture over a cation-exchange resin in a multistage series reactor. The reaction temperature in the individual reactors is below 65° C. Part of the intermediate product from the first reactor is recirculated to the inlet of the same reactor. The recirculation ratio (the amount of intermediate product mixture which is recirculated to the first reactor divided by the amount of feed mixture) is from 1.8 to 10 and the proportion by weight of tert-butyl alcohol in the feed mixture of tert-butyl alcohol and hydrocarbon mixture (sum of isobutene and any other hydrocarbons) at the inlet of the first reactor is from 0.5 to 3.5. The part of the mixture from the first reactor which has not been recirculated flows without intermediate introduction of water through two further reactors in a single pass. The crude product from the last reactor is worked up by distillation. If desired, part of the tert-butanol obtained is recirculated to the first reactor.
A disadvantage of this process is the low space-time yield. The production rate of tert-butanol in the first reactor (operation with product recirculation) is, according to examples 1 to 6, from 0.078 to 0.085 kg per hour per liter of catalyst (at an isobutene concentration at the inlet of from 8.3 to 16.2% by mass and conversions of from 60.2 to 66.1%).
DE 30 25 262 describes a process for preparing TBA using a three-component mixture of water/isobutene/TBA.
The process is preferably carried out at or nearby the border of the miscibility gap of this system, but can also be carried out in a heterogeneous and homogeneous region around the miscibility gap. The working range of the process is described by a triangular diagram, and there is wide variability of the composition of the mixture both in the heterogeneous and homogeneous phases because of the ternary mixture.
The compositions of this mixture in the homogeneous, i.e. single-phase region begin at the compositions having the maximum water content (solubility limit of water in the mixture, achievable by means of water separators) and end at mixtures having a low water content (less than about 5% based on the solubility limit).
The ternary mixture of isobutene-containing hydrocarbon/water/tert-butanol having a varying proportion of isobutene in the hydrocarbon mixture therefore leaves open, with the aid of DE 30 25 262, a large number of possible combinations for preparing tert-butanol highly selectively and at satisfactory reaction rates in the presence of a homogeneous catalyst. However, to obtain high reaction rates and selectivities, mixtures whose water contents are close to the solubility limit are preferably used in DE 30 25 262.